In order for such devices to be authorized for use in accordance with regulations, it must be ensured and plausibly established that the values measured by the laser scanner are correct on the one hand and, on the other hand, the measured values must be able to be assigned unequivocally to the appropriate vehicle.
DE 10 2009 007 055.9 is directed to a method which solves the problem of unequivocal association, in particular the association of a measured speed of a vehicle with a vehicle pictured in a graphic document.
In a method according to DE 10 2009 007 055.9, a measuring beam is scanned horizontally over a roadway over a given measurement angle area so that vehicles traveling through the resulting scan plane generate measured values from which the speed of the vehicles and the change in position over time can be determined.
The measuring beam is a pulsed laser beam which repeatedly scans the horizontal scan plane at a constant, predetermined scanning frequency and a constant, predetermined pulse frequency. For this purpose, a laser scanner is arranged next to the roadway at an acute angle to the direction of the roadway.
During the scan, the laser beam impinges on vehicles located within the measurement angle area and is reflected. The respective point of impingement is described by a measurement point Pn.
The transit time of the pulsed laser beam (pulse travel time) to and from the appropriate vehicle correlates to the distance traveled, from which the distance en of a measurement point Pn from the laser scanner positioned at the edge of the roadway can be derived and associated with a time tn, and an instantaneous scanning angle εn.
Using the measured values for the distance en, the instantaneous scanning angle εn and the time tn, the measurement points Pn can be described with a temporal correlation in a polar coordinate system (scanner coordinate system) which is determined by the position and orientation of the laser scanner.
After the speed of the appropriate vehicle has been determined from the measured values, this speed is compared to a given speed limit and, if this speed limit is exceeded, a digital camera is triggered to make an electronic recording as soon as this vehicle is located at a predetermined distance—the photo distance efoto—from the digital camera. The electronic recording is stored so that photographic evidence can be printed out at any time.
The digital camera is connected to the laser scanner by a controlling and evaluating unit and is basically oriented and adjusted in relation to the laser scanner and roadway such that it images an object field with a width of the angular area in-focus at the photo distance efoto.
A marker is generated from the measured values to visibly mark the appropriate vehicle as such in the photographic evidence. To this end, the measured values obtained within the scan closest in time to the triggering time of the digital camera are stored. The measured values of the scan during which the photo distance efoto was also established are advantageously stored.
The measured values of a scan consist of many pairs of measured values from an instantaneous scanning angle εn and a distance en, each representing a measurement point Pn. At a constant scanning frequency and a given pulse frequency, the vehicle surface or, more exactly, the front of the vehicle and a side of the vehicle as they are “seen” by the scanner causes, along a line in the scan plane, a finite quantity of measured value pairs which describe an angle-forming pair of lengths.
All of the measured value pairs which, taken together, describe an angle-forming length pair (hereinafter referred to as object angle) form a group that is generated in each instance by a vehicle. It will be clear to the person skilled in the art that the measurement points Pn determined by the measured value pairs lie only approximately on a straight line in practice, and the object angle is placed in the group of measured value pairs by means of mathematical approximation methods.
According to DE 10 2009 007 055.9, a marker is generated from the group of measured value pairs and is superimposed on the photographic recording so as to form a picture in which a marker occurs on the appropriate vehicle along the measurement points along the width and/or length of the imaged vehicle, preferably at the height of the scan plane.
Since the marker is formed on the basis of the measured value pairs of the scan which is carried out immediately upon establishing the photo distance efoto, it can be safely assumed that the vehicle whose position is at the photo distance efoto at the moment that the digital camera is triggered is in fact the appropriate vehicle.
In order to generate this marker, the group of measured value pairs representing measurement points Pn at the vehicle in the horizontal scan plane is transformed into a group of image points which represent correlating points in the image of the vehicle which is photographed in perspective, i.e., in the recording. A digitized marker containing the image points is then formed from the group of image points.
The electronically acquired image data of the vehicle which are obtained with the recording can be stored as a digitized recording in a file together with the digitized marker so that the marker is visibly superimposed on the appropriate vehicle when the file is graphically rendered.
However, it is more advantageous to store the digitized recording and the digitized marker in separate files which are, however, linked. For graphic rendering, the files are superimposed so that, in this case also, the marker is visibly superimposed on the appropriate vehicle. When stored separately, the digitized image of the vehicle, particularly of the license plate, remains unaffected and with its information fully intact.
The marker is then only temporarily superimposed on the recording of the vehicle for viewing the recording on a screen or for printing out a photograph.
By transforming the group of measured value pairs into a group of image points, a marker can be generated which is superimposed on the image of the vehicle so as to exactly cover the measurement points Pn which were correct immediately upon the triggering of the digital camera. This means that the marker is formed on the imaged vehicle along the points of impingement.
The marker advantageously covers all of these measurement points Pn and extends as a line completely along the length and width of the visible sides of the imaged vehicle.
However, it can also cover only some of the measurement points Pn and then advantageously forms a line extending only along the visible side on which the license number is not located so as to safely avoid affecting the image of the license number.
The certainty that the marked vehicle is also the appropriate vehicle is provided in that the measurement data used for marking the vehicle are brought about by the vehicle itself at the moment that an electronic recording of the appropriate vehicle is made.
In the method described in DE 10 2009 007 055.9, it is implied that the correlation between the scanner coordinate system and the camera coordinate system is known in order to allow the measurement data of the laser scanner system to be associated exactly with the image points in the recording of the digital camera which represent an image of the object points which are described as measurement points (points of impingement) by the measurement data. In other words, a marker generated from the measurement data can only be correctly transformed into the image when the spatial relationship between the two coordinate systems is known.
The object of the method according to the invention is to produce a spatial relationship of this kind.
A laser scanner and a digital camera which are to function jointly in an apparatus in coordination with one another are usually adjusted relative to one another when the apparatus is assembled by the manufacturer. To this end, the axis of the laser scanner around which the laser beam is deflected at a constantly changing scanning angle and the optical axis of the digital camera are adjusted relative to one another by means of a stationary measuring body in such a way that they “see” the measuring body at the same viewing angle. Ideally, this is only the case when the two axes coincide, although this is impossible in practice. Therefore, the axes are oriented parallel to one another at the least possible, negligible axial distance with respect to the measuring distance determined by the photo point. The measured values of the individual measurement points can then be converted into a Cartesian coordinate system and transformed into the associated image points taking into account the imaging characteristic of the digital camera, particularly the focal length of the camera objective.
It is disadvantageous that the laser scanner and the digital camera must be stabilized with respect to one another as the result of alignment. In arrangements in which the laser scanner and the digital camera are to be installed separately in the field, which is particularly advantageous when a plurality of roadways are to be monitored, prior alignment is not possible.
In contrast to the method known from DE 10 2009 007 055.9, where the laser scanner scans a monitoring area horizontally, the scanner can also scan the monitoring area vertically or also at an angle of inclination between 0° and 90°, e.g., in order to acquire the vehicle profile. Particularly in the case of vertical scanning, it is clear that it is not always useful to orient the laser scanning axis and the camera axis along the same axis. In general, the digital camera should be oriented in such a way at a height above the roadway and in the direction of traffic that the vehicle driver is photographed from the front.
However, on-site alignment has proven to be extremely cumbersome and also dangerous under certain circumstances, particularly when the equipment is installed, for example, at the edge of the road in moving traffic and the laser scanner and digital camera are not installed at the same location, e.g., when the laser scanner is installed to the side at the edge of the road and the digital camera is mounted on a bridge.